The present invention relates to a brake fluid pressure control apparatus for a vehicle, and more particularly to a brake fluid pressure control apparatus for a vehicle, which controls brake to prevent rollover of the vehicle.
As disclosed in JP2001-509448A (hereinafter referred to as Patent Literature 1), there is known a technique for preventing rollover of a vehicle by applying a braking force to at least one wheel to stabilize the vehicle when a driver steers the vehicle into a sharp turn and the vehicle is about to roll over.
As disclosed in JP10-081215A (hereinafter referred to as Patent Literature 2), there is known a technique for preventing rollover of a vehicle by applying braking forces only to turning outside wheels (i.e., wheels which are on the outside during cornering) to stabilize the vehicle when a driver steers the vehicle into a sharp turn and the vehicle is about to roll over, so that a lateral friction between the road surface and the tires can be reduced.
A vehicle is liable to roll over due to counteraction of roll, particularly when the driver steers back the steering wheel of a vehicle which is turning in one direction to the opposite direction so as to cause the vehicle to turn in the opposite direction. For the purpose of stabilizing the vehicle after such a steering-back maneuver, JP2007-513002A (hereinafter referred to as Patent Literature 3) discloses a technique for applying preparatory brake to turning inside wheels (I.e., wheels which are on the inside during cornering) after the driver executes the steering-back maneuver.
<First Drawback>
According to the technique disclosed in Patent Literature 1, an accelerometer for measuring a lateral acceleration, a measuring instrument for measuring a roll angle of the vehicle, and other instruments are used as sensors for detecting whether or not the vehicle is likely to roll over.
However, measuring physical quantities such as a lateral acceleration and a roll angle, which are equivalent to the actual roll angle of the vehicle, and then determining whether or not the vehicle is likely to roll over based on the measurement result may disadvantageously result in a drawback that the rollover prevention control will not sufficiently act on the vehicle if a rollover tendency of the vehicle increases more abruptly than normal. For example, if the driver of the vehicle executes an abrupt steering maneuver or a steering-back maneuver while the vehicle is turning, the vehicle is liable to roll over due to rolling-back of the vehicle generated after the steering maneuver. It would be desirable to provide a brake fluid pressure control apparatus for a vehicle, which can determine the rollover tendency of the vehicle at a timing as early as possible to promptly initiate the rollover prevention control.
<Second Drawback>
According to the technique disclosed in Patent Literature 1, the rollover tendency of the vehicle is detected by comparing measurements of various sensors indicating the roll angle or a measurement of a lateral acceleration sensor which changes with the same phase as the actual roll angle with predetermined threshold values.
However, the liability to cause the vehicle to roll over highly depends on whether the roll angle is gradually increasing or abruptly increasing, even if the roll angle takes the same value. Therefore, if the rollover tendency is detected only by determining whether a value equivalent to the roll angle reaches the predetermined threshold value and the rollover prevention control is initiated based on this determination, it is impossible to effectively perform the rollover prevention control in the case where the roll angle abruptly increases and the vehicle is more liable to roll over.
<Third Drawback>
According to the techniques disclosed in Patent Literatures 2 and 3, it is impossible to promptly increase braking forces at the wheels which are on the outside after the steering-back maneuver (i.e., wheels which are on the inside before the steering-back maneuver), when the driver makes a steering-back maneuver while the vehicle is turning. The technique disclosed in Patent Literature 3 is better than that disclosed in Patent Literature 2 because preparatory brake is applied alter the steering-back maneuver. However, it is after the steering-back maneuver that the preparatory brake is applied to the turning inside wheels, and therefore the braking forces at the wheels which are on the outside after the steering-back maneuver do not always increase sufficiently. Further, in this technique, the preparatory brake is applied for a short period of time, which contributes little to reducing the vehicle speed that is one of important factors to prevent rollover of the vehicle.
In view of the above, it would be desirable to provide a brake fluid pressure control apparatus, which can overcome one or more of the above drawbacks and improve the stability while driving the vehicle.